[PATCH 1/6] rcu: Remove full ordering on second EQS snapshot

Frederic Weisbecker
2024-05-15T12:53:40Z

When the grace period kthread checks the extended quiescent state
counter of a CPU, full ordering is necessary to ensure that either:

* If the GP kthread observes the remote target in an extended quiescent
state, then that target must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it exits that extended quiescent state. Also the GP kthread must
observe all accesses performed by the target prior it entering in
EQS.

or:

* If the GP kthread observes the remote target NOT in an extended
quiescent state, then the target further entering in an extended
quiescent state must observe all accesses prior to the current
grace period, including the current grace period sequence number, once
it enters that extended quiescent state. Also the GP kthread later
observing that EQS must also observe all accesses performed by the
target prior it entering in EQS.

This ordering is explicitly performed both on the first EQS snapshot
and on the second one as well through the combination of a preceding
full barrier followed by an acquire read. However the second snapshot's
full memory barrier is redundant and not needed to enforce the above
guarantees:

GP kthread Remote target
---- -----
// Access prior GP
WRITE_ONCE(A, 1)
// first snapshot
smp_mb()
x = smp_load_acquire(EQS)
// Access prior GP
WRITE_ONCE(B, 1)
// EQS enter
// implied full barrier by atomic_add_return()
atomic_add_return(RCU_DYNTICKS_IDX, EQS)
// implied full barrier by atomic_add_return()
READ_ONCE(A)
// second snapshot
y = smp_load_acquire(EQS)
z = READ_ONCE(B)

If the GP kthread above fails to observe the remote target in EQS
(x not in EQS), the remote target will observe A == 1 after further
entering in EQS. Then the second snapshot taken by the GP kthread only
need to be an acquire read in order to observe z == 1.

Therefore remove the needless full memory barrier on second snapshot.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5e6828132007..58415cdc54f8 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -325,7 +325,7 @@ static bool rcu_dynticks_in_eqs(int snap)
*/
static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
{
- return snap != rcu_dynticks_snap(rdp->cpu);
+ return snap != ct_dynticks_cpu_acquire(rdp->cpu);
}

/*
--
2.44.0